/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/Mechanics/RectangleMarqueeMechanic.h
OnDragRectangleChanged is called as the user drags the other corner of the rectangle around.
typedef TMulticastDelegate_OneParam< void, const [FCameraRectangle](API\Plugins\ModelingComponents\Mechanics\FCameraRectangle) & > OnDragRectangleChangedEvent